Electrical terminals and connectors are typically made of high conductivity metals such as copper or a variety of alloys thereof including brass, phosphor bronze, beryllium copper and the like. For most low voltage applications and certain high voltage applications it is necessary to protect the surfaces of such articles by providing a film, coating or plating deposited on the surfaces thereof to minimize the buildup in time of unwanted oxides which may interfere with electrical interface. The most common practice is to utilize commercial and proprietary plating solutions of copper, nickel, zinc, tin, gold and in some cases tin lead alloys. Typically, the process involves dissolving the chosen metal to be plated into solution through the use of an anode, cathode, electrolyte and currents and voltages to effect electro deposition. Plating by electro deposition involves numerous steps of mechanical polishing, cleaning, buffing, degreasing, descaling, and a variety of electrolytic and chemical polishing steps. Following plating various steps of rinsing and further cleaning are necessary with substantial quantities of water, heat for the baths and plating stations and a variety of additive chemicals which effect efficiency of the plating process and help characterize the micro structure of the plating itself. The typical plating plant is quite large in terms of use of volume and quite costly in terms of equipment required. It is not unknown for the pollution equipment associated with plating plants to equal or exceed the amount of space required and the cost of the actual plating equipment.
Other techniques for coating have evolved including the process of sputtering, a process which is required to be done at modest vacuum but which because of the nature of the process produces relatively slow rates of deposition and the energy required in the sputtering process is very high.
With respect to the kinds of coatings required by electrical contacts most deposits vary in accordance with the design life, environment expected, number of cycles of engagement in use, and a variety of other such factors. Depositions on the order of a few millionths of an inch of gold are used for cosmetic purposes with actual contact surfaces ranging from 15 to 80 millionth of an inch and occasionally 100 or 125 millionth of an inch for certain critical specifications. Depositions of nickel, typically as barrier coatings beneath gold finishes and/or tin finishes range in coating thickness on the order of 100 millionths of an inch. Tin platings, and tin lead platings and various alloys thereof which are utilized most often relative to solder operations for terminals, are typically on the order of 240 millionths of an inch in thickness.
With respect to electrical terminals and connectors, the volumes requiring coating can for given installation number in the hundreds of thousands or millions per plant site per shift. This requirement means that whatever facility or processes used must be capable of high volume production of relatively tightly toleranced plating thicknesses.
Accordingly, it is an object of the invention to provide a method and apparatus for practicing such method that achieves a coating in the form of a variety of metal finishes needed for metal articles such as electrical terminals and connectors that can be operated at relatively high speeds to relatively tight tolerances and which in terms of space and cost represents a substantial improvement over existing electroplating practices. It is a further object to provide a novel dry plating method and apparatus having fewer steps than with other known practices of deposition and which may be practiced in a relatively small space essentially without the need for vast volumes of water and attendant pollution control equipment. It is yet a further object of the invention to provide a novel and effective coating as a finish for metal articles in the form of narrow strips including particularly electrical terminals and connectors yielding a novel microstructure having appropriate hardness and yet ductility when compared with other techniques of deposition.